Vehicle manufacturers have been diligently working to develop alternative powertrain systems in an effort to reduce the level of pollutants exhausted into the air by conventional powertrains equipped with internal combustion engines and also to increase the fuel efficiency thereof. Significant development has been directed to hybrid vehicles. Several different hybrid electric vehicles are configured with an internal combustion and an electric motor that may be operated independently or in combination to drive the vehicle.
In general, there are two types of hybrid vehicles, namely, series hybrid and parallel hybrid. In a series hybrid vehicle, power is delivered to the wheels by the electric motor which draws electrical energy from a battery. The engine is used in a series hybrid vehicle to drive a generator which supplies power directly to the motor or charges the battery when a state of charge falls below a predetermined value. In a parallel hybrid vehicle, the electric motor and the internal combustion engine may be operated independently or in combination with one another.
In one known hybrid vehicle powertrain, the valve train of the internal combustion engine is controlled such that air is not compressed during a compression stroke of the internal combustion engine when power is provided solely from the electric motor. While this configuration may function properly, such an arrangement may be very costly and complex. As such, it may be desirable to provide a simplified hybrid drive system.